Synchronization symbol structure using OFDM based transmission method

ABSTRACT

The present invention proposes a method for generating synchronization bursts for OFDM transmission systems. The symbols of a predefined symbol sequence are mapped according to a predefined mapping scheme on subcarriers of the OFDM systems by a mapping unit ( 2 ), wherein the symbols of the predefined symbol sequence represent subcarriers of the OFDM system with nonzero amplitudes. A synchronization burst is generated by a inverse fast Fourier transforming unit ( 3 ) transforming the subcarriers of the OFDM system mapped to said predefined symbol sequence. The mapping ( 2 ) of the symbols of the predefined symbol sequence is set such that the resulting time domain signal of the synchronization burst represents a periodic nature. According to the invention the predefined symbol sequence is set such that the envelope fluctuation of the time domain signal of the synchronization burst is minimized. Therefore advantageous symbol sequences reducing said the envelope fluctuation of the time domain signal are proposed.

Notice: More than one application has been filed for the reissue of U.S.Pat. No. 6,654,339. The reissue applications are Ser. No. 11/284,440,the instant application and application Ser. Nos. 12/258,799,12/258,984, 12/259,018, 12/259,045 and 12/259,063, all filed Oct. 27,2008.

The present invention relates to a method for generating synchronizationbursts for OFDM transmission systems, a method for synchronizingwireless OFDM systems, an OFDM transmitter as well as to a mobilecommunications device comprising such a transmitter.

The present invention relates generally to the technical field ofsynchronizing wireless OFDM (orthogonal frequency division multiplexing)systems. Thereby it is known to use a synchronization burst constructedusing especially designed OFDM symbols and time domain repetitions.

Particularly from the document IEEE P802.11a/d2.0 “Draft supplement to astandard for telecommunications and information exchange betweensystems—LAN/MAN specific requirements—part 1: wireless medium accesscontrol (MAC) and physical layer (PHY) specifications: high-speedphysical layer in the 5 GHz band” a synchronization scheme for OFDMsystems is proposed. This document is herewith included by reference asfar as it concerns the synchronization including the proposedimplementation. Said known scheme will now be explained with referenceto FIG. 6 to 8 of the enclosed drawings.

FIG. 6 shows the structure of the known synchronization field. As shownin FIG. 6 the synchronization field consists of so-called short symbolst1, t2, . . . t6 and two long symbols T1, T2. In view of the presentinvention particularly the short symbols t1, t2 . . . t6 are ofinterest. Among the short symbols t1 t2, . . . t6 used for the amplifiergain control (t1, t2, t3) and the course frequency offset and timingcontrol only the symbols t1, t2, t3 and t4 are actually generated,whereas the symbols t5, t6 are cyclic extensions (copies of the symbolst1 and t2, respectively). It is to be noted that FIG. 5 shows only thesynchronization preamble structure as the structure of the followingsignal field indicating the type of baseband modulation and the codingrate as well as the structure of further following data fields are notof interest in view of the present invention. For further detailsreference is made to said prior art document.

The symbols t1, t2, t3, t4 are generated by means of an OFDM modulationusing selected subcarriers from the entire available subcarriers. Thesymbols used for the OFDM modulation as well as the mapping to theselected subcarriers will now be explained with reference to FIG. 6.

Each of the short OFDM symbols t1, . . . t6 is generated by using 12modulated subcarriers phase-modulated by the elements of the symbolalphabet:

-   -   S= 2(±1±j)

The full sequence used for the OFDM modulation can be written asfollows:

-   -   S_(−24,24)=√2*{1+j,0,0,0,1+j,0,0,0,−1−j,0,0,0,−1−j,0,0,0,1−j,0,0,0,−1−j,0,0,0,0        0,0,0,1+j,0,0,0,1+j,0,0,0,−1−j,0,0,0,1+j,0,0,0,−1+j,0,0,0,1+j}

The multiplication by a factor of √2 is in order to normalize theaverage power of the resulting OFDM symbol.

The signal can be written as:${r_{SHORT}(t)} = {{w_{SHORT1}(t)}{\sum\limits_{k = {{- N_{2}}\text{/}2}}^{N_{s}\text{/}2}\quad{S_{k}{\exp\left( {{j2\pi}\quad{k\Delta}_{F}t} \right)}}}}$

The fact that only spectral lines of S_(−24,24) with indices which are amultiple of 4 have nonzero amplitude results in a periodicity ofT_(FFT)/4=0.8 μsec. The interval T_(TSHORT1) is equal to nine 0.8 μsecperiods, i.e. 7.2 μsec.

Applying a 64-point IFFT to the vector S, where the remaining 15 valuesare set to zero, four short training symbols t1, t2, t3, t4 (in the timedomain) can be generated. The IFFT output is cyclically extended toresult in 6 short symbols t1, t2, t3, . . . t6. The mapping scheme isdepicted in FIG. 7. The so called virtual subcarriers are leftunmodulated.

The way to implement the inverse Fourier transform is by an IFFT(Inverse Fast Fourier Transform) algorithm. If, for example, a 64 pointIFFT is used, the coefficients 1 to 24 are mapped to same numbered IFFTinputs, while the coefficients −24 to −1 are copied into IFFT inputs 40to 63. The rest of the inputs, 25 to 39 and the 0 (DC) input, are set tozero. This mapping is illustrated in FIG. 7. After performing an IFFTthe output is cyclically extended to the desired length.

With the proposed inverse fast Fourier transform (IFFT) mapping as shownin FIG. 7 the resulting time domain signal consists of 4 periodicallyrepeated short symbols t1, t2, t3, t4, and cyclically extended by a copyof t1, t2, which copy is depicted in FIG. 5 as t5, t6. Note that in thepresent case only spectral lines with indices which are a multiple of 4have nonzero amplitude. Other periodic natures can be generated bysetting other multiples of the spectral lines to nonzero amplitudes.

Though the known synchronization scheme is very effective, it providesfor disadvantage regarding the time domain signal properties.

For OFDM (or in general multicarrier signals) the signal envelopefluctuation (named Peak-to-Average-Power-Ratio=PAPR) is of greatconcern. A large PAPR results in poor transmission (due to nonlineardistortion effects of the power amplifier) and other signal limitingcomponents in the transmission system (e.g. limited dynamic range of theAD converter).

For synchronization sequences it is even more desirable to have signalswith a low PAPR in order to accelerate the receiver AGC (automatic gaincontrol) locking and adjusting the reference signal value for the A/Dconverter (the whole dynamic range of the incoming signal should becovered by the A/D converter resolution without any overflow/underflow).

FIGS. 8a, 8b show the “absolute” (sqrt{In*+Quad*Quad}) value of theresulting time domain signal waveform with the sequences proposed byLucent Technologies. Oversampling (8*) was considered in order to ensurethe peak was captured correctly using the limited 64-point IFFT.

FIGS. 8c, 8d show the real and imaginary part of the resultingtransmitted time domain waveform. The resulting PAPR is 2.9991 dB (nooversampling) and 3.0093 dB (with 8 times oversampling).

Therefore it is the object of the present invention to provide for asynchronization technique which bases on the known synchronizationtechnique but which presents improved time domain signal properties toreduce the requirements for the hardware.

The above object is achieved by means of the features of the independentclaims. The dependent claims develop further the central idea of thepresent invention.

According to the present invention therefore a method for generatingsynchronization bursts for OFDM transmission systems is provided.Symbols of a predefined symbol sequence are mapped according to apredefined mapping scheme on subcarriers of the OFDM system wherein thesymbols of the predefined symbol sequence represent subcarriers withnonzero amplitudes. A synchronization burst is generated by inverse fastFourier transforming the subcarriers mapped with a predefined symbolsequence. According to the present invention the predefined symbolsequence is optimized such that the envelope fluctuation of the timedomain signal (Peak-to-average-power-ratio) is minimized.

The predefined symbol sequence can be chosen such that the followingequations are satisfied for all symbols of the predefined symbolsequence:n=2m,C_(i−1)=±C_(1−i),

-   -   n being the number of symbols of the predefined symbol sequence,    -   m being an integer larger than one,    -   C being the symbol value, and    -   i being an integer running from 1 to m.

The mapping of the symbols of the predefined symbol sequence and theInverse Fast Fourier Transform can be set such that the resulting timedomain signal of the synchronization burst represents a periodic nature.

Alternatively the mapping of the symbols of the predefined symbolsequence and the Inverse Fast Fourier Transform is set such that oneburst part of the synchronization burst in the time domain is generatedand the periodic nature of the synchronization burst in the time domainis achieved by copying the one burst part.

The number of symbols of a symbol sequence (n) can for example be 12.

The above equations define generally the symbol sequences according tothe present invention. The predefined symbol sequence can therefore befor example:

-   -   A A A -A -A -A -A A -A -A A -A,        wherein A is a complex value.

Alternatively the predefined symbol sequence can be:

-   -   A -A A A -A A A A A -A -A -A,        wherein A is a complex value.

Alternatively the following predefined symbol sequence can be used:

-   -   A B -A B -A -B B A -B A -B -A,        wherein A, B are complex values.

As a further alternative the following sequence can be used:

-   -   A -B -A -B -A B -B A B A B -A,        wherein A, B are complex values.

According to the present invention furthermore a method forsynchronizing wireless OFDM systems is provided, wherein asynchronization burst is generated according to a method as set forthabove and the synchronization burst is transmitted respectively beforethe transmission of data fields.

Thereby the time domain signals of the synchronization burst can beprecomputed and stored in a memory, such that the computation of thetime domain signal of the burst is only effected once.

According to the present invention furthermore a OFDM transmitter isprovided comprising a mapping unit for mapping the symbols of apredefined symbols sequence according to a predefined mapping scheme onsubcarriers of the OFDM system, wherein the symbols of a predefinedsymbols sequence represent the subcarriers of the OFDM system withnonzero amplitudes. Furthermore an inverse fast Fourier transformingunit is provided for generating a synchronization burst by inverse fastFourier transforming the subcarriers of the OFDM mapped with saidpredefined symbols sequence. The mapping unit thereby is designed suchthat the resulting time domain signal of the synchronization burstrepresents a periodic nature. The mapping unit according to the presentinvention uses a predefined symbol sequence which is such that theenvelope fluctuation of the time domain signal of the synchronizationburst is minimized.

According to the present invention furthermore a mobile communicationsdevice such as set forth above is used.

With reference to the figures of the enclosed drawings referredembodiments of the present invention will now be explained.

FIG. 1 shows schematically a transmitter according to the presentinvention,

FIG. 2 shows an alternative embodiment for a transmitter according tothe present invention,

FIG. 3 shows an alternative mapping scheme according to the presentinvention,

FIGS. 4a to 4d show the time domain signal properties achieved with thesynchronization symbol structure using OFDM based transmission accordingto the present invention,

FIGS. 5a to 5d show the time domain signal properties of synchronizationsymbol structures according to alternative embodiments of the presentinvention,

FIG. 6 shows a synchronization preamble structure known from the priorart,

FIG. 7 shows an IFFT mapping according to the prior art, and

FIGS. 8a to 8d show the time domain properties of the synchronizationsymbol structure according to the prior art,

FIGS. 9a and 9b show the time domain properties, particularly thedynamic range of the synchronization symbol structure according to theprior art, and

FIGS. 10a and 10b show the time domain properties of the synchronizationsymbol structure according to further alternative embodiments of thepresent invention,

According to the present invention the time domain synchronization burststructure as shown in FIG. 6 is maintained. The IFFT mapping as shown inFIG. 7 can be maintained or alternatively the IFFT mapping according toFIG. 3 can be used. The symbol sequences mapped to the subcarriers areoptimized to sequences which result in a lower PAPR.

According to the present invention a short OFDM symbol (t1, . . . t6)consists of 12 phase-modulated subcarriers.

C00 C01 C02 C03 C04 C05 C06 C07 C08 C09 C10 C11 Seq0 A   A   A −A −A −A−A A −A −A   A −A Seq1 A −A   A   A −A   A   A A   A −A −A −A Seq2 A   B−A   B −A −B   B A −B   A −B −A Seq3 A −B −A −B −A   B −B A   B   A   B−Awith A = exp (j * 2 + π * φ_(A))  and$B = {{A*{\exp\left( {j\frac{\pi}{2}} \right)}} = {{{\exp\left( {{{j2\pi}*\varphi_{A}} + {j\frac{\pi}{2}}} \right)}\quad{and}\quad 0.0} \leq \varphi_{A} < {1.0.}}}$

Generally the predefined symbol sequence therefore is chosen such thatthe envelope fluctuation of the time domain signal of thesynchronization burst is minimized.

Therefore generally the predefined symbol sequence is set such that thefollowing equations are satisfied for all symbols for the predefinedsymbol sequence:n=2m,C_(i−1)=±C_(n−i)

-   -   wherein n is a number of symbols of the predefined symbol        sequence,    -   m is an integer larger than 1,    -   c is the symbol value, and    -   i is an integer value running from 1 to m.

In the following the time domain signal properties of the new sequencesaccording to the present invention will be shown with reference to FIGS.4a to 4d and FIGS. 5a to 5d.

For simplicity we use in our demonstration the classical quadriphasesymbol alphabet,${S = {\sqrt{\frac{1}{2}}\left( {{\pm 1} \pm j} \right)}},$(this corresponds to φ_(A)=0.125)

Symbol   A $\exp\left( {j\frac{\pi}{4}} \right)$$\sqrt{\frac{1}{2}}\left( {{+ 1} + j} \right)$ −A${- {\exp\left( {j\frac{\pi}{4}} \right)}} = {\exp\left( {j\frac{5\pi}{4}} \right)}$$\sqrt{\frac{1}{2}}\left( {{- 1} - j} \right)$   B${\exp\left( {{j\frac{\pi}{4}} + {j\frac{\pi}{2}}} \right)} = {\exp\left( {j\frac{3\pi}{4}} \right)}$$\sqrt{\frac{1}{2}}\left( {{- 1} + j} \right)$ −B${- {\exp\left( {j\frac{3\pi}{4}} \right)}} = {\exp\left( {j\frac{7\pi}{4}} \right)}$$\sqrt{\frac{1}{2}}\left( {{+ 1} - j} \right)$Table 1: Complex symbol mapping

FIGS. 5a and 5b thereby show the time domain signal (magnitude) whenusing the optimized sequence according to the present invention in thecase of no oversampling/8-times oversampling is effected.

PAPR (in decibel) is limited to 2.059 (even when using a time domainoversampling to capture the actual peak).

FIGS. 5c and 5d show the in-phase and quadrature-phase component,respectively, of the resulting wave form. It is clearly visible that thefull symbol consists of four repetitions of a short sequence.

FIGS. 5a to 5d show graphics corresponding to FIGS. 4a to 4d for theother proposed sequences S1, S2 and S3.

Further simulations have shown that not only the PAPR can be optimizedbut also the dynamic range of the signal should be minimized. Thereforeanother four sequences, with achieve a small PAPR and at the same time asmall overall dynamic range are proposed further below.

Using the sequence as proposed in the state of the art the PAPR is 3.01dB and the dynamic range (defined as the ratio of the peak power to theminimum power) is 30.82 dB (see FIGS. 9a and 9b).

Using the sequences according to the present invention and as describedabove the PAPR is reduced to 2.06 dB, however, the dynamic range isincreased as the signal power is ‘0’ at some points.

Therefore the following four sequences are proposed as a furtherembodiment of the present invention:

The symbol sequence is C0, C1, . . . C11 and the mapping is:

-   -   S=2*{C00, 0, 0, 0, C01, 0, 0, 0, C02, 0, 0, 0, C03, 0, 0, 0,        C04, 0, 0, 0, C05, 0, 0, 0, 0, 0, 0, 0, C06, 0, 0, 0, C07, 0, 0,        0, C08, 0, 0, 0, C09, 0, 0, 0, C10, 0, 0, 0, C11}

C00 C01 C02 C03 C04 C05 C06 C07 C08 C09 C10 C11 Seq-Alt0 A   A   A   A−A −A   A −A −A   A −A A Seq-Alt1 A −A   A −A −A   A −A −A   A   A   A ASeq-Alt2 A   B −A −B −A −B −B −A −B −A   B A Seq-Alt3 A −B −A   B −A   B  B −A   B −A −B Awith A=exp (i*2*π*φ_(A)) and$B = {{A*{\exp\left( {j\frac{\pi}{2}} \right)}} = {\exp\left( {{{j2\pi}*\varphi_{A}} + {j\frac{\pi}{2}}} \right)}}$and 0.0≦φ_(A<)1.0.

Using these sequences the PAPR is reduced to 2.24 dB and the dynamicrange is limited to 7.01 dB as it is shown in FIGS. 10a and 10b.

The advantages are the same as described before, however, the clippingproblem is further reduced due to the very limited dynamic range of thesignal.

With reference to FIG. 1 and 2 possible implementations of a transmitteraccording to the present invention will now be explained.

In the transmitter the sync symbol data 1 are prepared and mapped in aIFFT mapping unit 2 to the appropriate IFFT points. The subcarriers ofthe OFDM system are transformed by a IFFT unit 3 and then the timedomain signal is extended in a time extension unit 4 by copying parts ofthe signals (for example, t1, t2 are copied to t5, t6). The timeextended signal is then sent to the I/Q modulator 5.

As shown in FIG. 2 alternatively the time domain signal can beprecomputed once in a computation unit 7 and then be stored in a memory6 for the precomputed sample for the time signal. Then the time domainsignal of the synchronization burst can be sent to the modulator 5directly from the memory 6.

With reference to FIG. 3 a modified IFFT mapping scheme will now beexplained.

According to this scheme, the principle of setting only every fourthsubcarrier of the OFDM system to a non-zero amplitude (see FIG. 7) isabandoned. Therefore the time domain signal achieved according to themapping scheme of FIG. 3 will not present a periodic nature.

The IFFT size is now only 16 (instead of 64 as it is the case in FIG.7). Only one of the bursts t1, t2, . . . t6 will be generated. The otherbursts can be generated by copying to retain the periodic nature of thesynchronization time domain signal necessary for the correlation andsynchronization on the receiving side. Therefore for example the timeextension unit 4 can perform the copying of the 16-sample burst t1generated by the IFFT 16 according to FIG. 7 to the other burst t2, t3,. . . t6. Obviously the mapping scheme according to FIG. 3 reduces thecomputing effort necessary for the IFFT. The periodic nature of the timedomain signal of the SYNCH bursts is therefore no longer achieved by theIFFT step, but by copying the burst t1 generated with the simplifiedIFFT mapping scheme.

The mapping scheme shown in FIG. 3 is also advantageous in combinationwith the precomputing technique shown in FIG. 2.

According to the present invention therefore a synchronization burststructure to be used in high speed wireless transmission systems isproposed. The synchronization burst is constructed using especiallydesigned OFDM symbols and time domain repetitions. The resultingsynchronization burst achieves a high timing detection and frequencyoffset estimation accuracy. Furthermore the burst is optimized toachieve a very low envelope fluctuation (Lowpeak-to-average-power-ratio) to reduce the complexity on the receiverand to reduce time and frequency acquisition time at the receiver.

Therefore the synchronization performance can further be improved. Aswith the scheme according to the present invention the envelope of theOFDM based synchronization burst in the time domain is reduced, the AGCpool-in speed at the receiver can be improved and an accurate time andfrequency synchronization can be achieved. Furthermore thesynchronization complexity on the receiver side can be reduced due tothe reduced resolution requirements necessary due to reduced envelopefluctuation.

The advantages of the present invention can be set forth as following:

-   -   An OFDM based SYNCH symbol with a reduced        Peak-to-Average-Power-Ratio (PARP) is proposed,    -   Improved synchronization performance (compared to the state of        the art proposal),    -   Reduced AGC (automatic gain control) pull-in time due to reduced        dynamic range of the SYNCH burst,    -   Improved AGC settlement (AGC has to adjust to a incoming signal        level that later on now overflow/underflow in the AD happens.        The reduced dynamic range of the SYNCH burst help to find this        reference level more accurate),    -   Reduced synchronization detection complexity on the receiver        (reduced resolution necessary due to reduced envelope        fluctuation).

1. A method for generating synchronization bursts for OFDM transmissionsystems, comprising the following steps: mapping the symbols of apredefined symbol sequence according to a predefined mapping scheme onsubcarriers S of the OFDM system, wherein the symbols of the predefinedsymbol sequence represent subcarriers of the OFDM system withnon-zero-amplitude, and generating a synchronization burst by InverseFourier Transforming the subcarriers S of the OFDM system mapped withthe symbols of said predefined symbol sequence, characterized in thatthe predefined symbol sequence is set such that the envelope fluctuationof the time domain signal of the synchronization burst is minimized andthe symbols of the predefined symbols sequence can be expressed as A -AA -A -A A -A -A A A A A A being a complex value.
 2. A method forsynchronizing wireless OFDM systems, characterized by the steps ofgenerating a synchronization burst according to a method according toclaim 1, and transmitting the synchronization burst.
 3. A methodaccording to claim 2, characterized in that the time domain signal ofthe synchronization burst is precomputed and stored in a memory.
 4. AnOFDM transmitter, comprising: a unit for mapping the symbols of apredefined symbol sequence according to a predefined mapping scheme onsubcarriers of the OFDM system, wherein the symbols of the predefinedsymbol sequence represent subcarriers of the OFDM system withnon-zero-amplitude, and a unit for generating a synchronization burst byInverse Fourier Transforming the subcarriers of the OFDM system mappedwith the symbols of said predefined symbol sequence, characterized inthat the mapping unit is designed to modulate the subcarriers such thatthe envelope fluctuation of the time domain signal of thesynchronization burst is minimized by using the following predefinedsymbol sequence: A -A A -A -A A -A -A A A A A A being a complex value.5. An OFDM transmitter according to claim 4, characterized by a timeextension unit copying the burst part to achieve a periodic nature ofthe time domain signal.
 6. An OFDM transmitter according to claim 4,characterized by a processing unit for precomputing the time domainsignal of the synchronization burst and a memory for storing theprecomputed time domain signal of the synchronization burst.
 7. A mobilecommunications device, comprising a transmitter according to claim
 4. 8.A synchronization burst signal for synchronizing OFDM systems generatedby a method according to claim
 1. 9. A method, utilizing a computationunit, for generating a synchronization signal by using a plurality ofsubcarriers for an OFDM transmission system, comprising the steps of:mapping symbols of a predefined symbols sequence in accordance with apredefined mapping scheme on said plurality of subcarriers, whereinpre-selected symbols of the predefined symbol sequence have non-zerovalues and have the value${\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} + j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} - j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} + j} \right)},\quad{{or}\quad\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} - j} \right)},$generating only one synchronization signal by Inverse FourierTransforming said plurality of subcarriers mapped with the symbols ofsaid predefined symbol sequence, and copying said one synchronizationsignal to generate other synchronization signals in the time domain. 10.A method, utilizing a computation unit, for generating a synchronizationsignal by using a plurality of subcarriers for an OFDM transmissionsystem, comprising the steps of: generating a predefined symbol sequencehaving a pre-selected number of symbols corresponding to respectivepre-selected ones of said plurality of subcarriers, wherein saidpreselelected symbols are set to non-zero complex values and others ofsaid symbols are set to zero, such that said pre-selected symbols arearranged periodically in said predefined symbols sequence in thefrequency domain, and have the value${\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} + j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} - j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} + j} \right)},\quad{{or}\quad\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} - j} \right)},$generating only one synchronization signal in time domain by performingInverse Fourier Transforming on said pre-selected ones of said pluralityof subcarriers, and copying said one synchronization signal to generateother synchronization signals in the time domain.
 11. A method,utilizing a computation unit, for generating a synchronization signal byusing a plurality of subcarriers for an OFDM transmission system,comprising the steps of: generating a predefined symbol sequence havingpre-selected symbols each set to a non-zero value and a plurality offurther symbols each set to a zero value, wherein respectivepre-selected symbols have the value${\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} + j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} - j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} + j} \right)},\quad{{or}\quad\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} - j} \right)},$ and wherein each of said symbols is mapped respectively on a predefinedsubcarrier of said plurality of subcarriers, generating only onesynchronization signal in time domain by performing Inverse FourierTransforming on said plurality of subcarriers mapped with saidpredefined symbols sequence, and copying said one synchronization signalto generate other synchronization signals in the time domain.
 12. Amethod, utilizing a computation unit, for generating a synchronizationsignal by using a plurality of subcarriers for an OFDM transmissionsystem, comprising the steps of: mapping symbols of a predefined symbolsequence in accordance with a predefined mapping scheme on saidplurality of subcarriers, wherein pre-selected symbols of the predefinedsymbol sequence have non-zero values and have the value${\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} + j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} - j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} + j} \right)},\quad{{or}\quad\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} - j} \right)},$generating only one synchronization signal by Inverse FourierTransforming said plurality of subcarriers mapped with the symbols ofsaid predefined symbol sequence, and copying said one synchronizationsignal in the time domain to provide a synchronization signal withperiodicity.
 13. A method, utilizing a computation unit, for generatinga synchronization signal by using a plurality of subcarriers for an OFDMtransmission system, comprising the steps of: generating a predefinedsymbol sequence having a pre-selected number of symbols corresponding torespective pre-selected ones of said plurality of subcarriers, whereinsaid preselected symbols are set to non-zero complex values and othersof said symbols are set to zero, such that said pre-selected symbols arearranged periodically in said predefined symbol sequence in thefrequency domain, and wherein respective pre-selected symbols have thevalue${\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} + j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} - j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} + j} \right)},\quad{{or}\quad\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} - j} \right)},$generating only one synchronization signal in time domain by performingInverse Fourier Transforming on said pre-selected ones of said pluralityof subcarriers, and copying said one synchronization signal in the timedomain to provide a synchronization signal with periodicity.
 14. Amethod, utilizing a computation unit, for generating a synchronizationsignal by using a plurality of subcarriers for an OFDM transmissionsystem, comprising the steps of: generating a predefined symbol sequencehaving pre-selected symbols each set to a non-zero value and a pluralityof further symbols each set to a zero value, wherein respective ones ofsaid pre-selected symbols have the value${\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} + j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} - j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} + j} \right)},\quad{{or}\quad\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} - j} \right)},$ and wherein each of said symbols is mapped respectively on a predefinedsubcarrier of said plurality of subcarriers, generating only onesynchronization signal in time domain by performing Inverse FourierTransforming on said plurality of subcarriers mapped with saidpredefined symbols sequence, and copying said one synchronization signalin the time domain to provide a synchronization signal with periodicity.15. Apparatus for generating a synchronization signal by using aplurality of subcarriers for an OFDM transmission system, comprising: aunit for mapping symbols of a predefined symbols sequence in accordancewith a predefined mapping scheme on said plurality of subcarriers,wherein pre-selected symbols of the predefined symbol sequence havenon-zero values and have the value${\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} + j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} - j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} + j} \right)},\quad{{or}\quad\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} - j} \right)},$a unit for generating only one synchronization signal by Inverse FourierTransforming said plurality of subcarriers mapped with the symbols ofsaid predefined symbol sequence, and a unit for copying said onesynchronization signal to generate other synchronization signals in thetime domain.
 16. Apparatus for generating a synchronization signal byusing a plurality of subcarriers for an OFDM transmission system,comprising: a unit for generating a predefined symbol sequence having apre-selected number of symbols corresponding to respective pre-selectedones of said plurality of subcarriers, wherein said preselected symbolsare set to non-zero complex values and others of said symbols are set tozero, such that said pre-selected symbols are arranged periodically insaid predefined symbol sequence in the frequency domain, and whereinrespective ones of said pre-selected symbols have the value${\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} + j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} - j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} + j} \right)},\quad{{or}\quad\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} - j} \right)},$a unit for generating only one synchronization signal in time domain byperforming Inverse Fourier Transforming on said pre-selected ones ofsaid plurality of subcarriers, and a unit for copying said onesynchronization signal to generate other synchronization signals in thetime domain.
 17. Apparatus for generating a synchronization signal byusing a plurality of subcarriers for an OFDM transmission system,comprising: a unit for generating a predefined symbol sequence havingpre-selected symbols each set to a non-zero value and a plurality offurther symbols each set to a zero value, wherein respective ones ofsaid pre-selected symbols have the value${\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} + j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} - j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} + j} \right)},\quad{{or}\quad\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} - j} \right)},$ and wherein each of said symbols is mapped respectively on a predefinedsubcarrier of said plurality of subcarriers, a unit for generating onlyone synchronization signal in time domain by performing Inverse FourierTransforming on said plurality of subcarriers mapped with saidpredefined symbols sequence, and a unit for copying said onesynchronization signal to generate other synchronization signals in thetime domain.
 18. Apparatus for generating a synchronization signal byusing a plurality of subcarriers for an OFDM transmission system,comprising: a unit for mapping symbols of a predefined symbols sequencein accordance with a predefined mapping scheme on said plurality ofsubcarriers, wherein respective ones of said pre-selected symbols of thepredefined symbol sequence have non-zero values${\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} + j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} - j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} + j} \right)},\quad{{or}\quad\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} - j} \right)},$a unit for generating only one synchronization signal by Inverse FourierTransforming said plurality of subcarriers mapped with the symbols ofsaid predefined symbol sequence, and a unit for copying said onesynchronization signal in the time domain to provide a synchronizationsignal with periodicity.
 19. Apparatus for generating a synchronizationsignal by using a plurality of subcarriers for an OFDM transmissionsystem, comprising: a unit for generating a predefined symbol sequencehaving a pre-selected number of symbols corresponding to respectivepre-selected ones of said plurality of subcarriers, wherein saidpreselected symbols are set to non-zero complex values and others ofsaid symbols are set to zero, such that said pre-selected symbols arearranged periodically in said predefined symbol sequence in thefrequency domain, and respective ones of said pre-selected symbols havethe value${\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} + j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} - j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} + j} \right)},\quad{{or}\quad\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} - j} \right)},$a unit for generating only one synchronization signal in time domain byperforming Inverse Fourier Transforming on said pre-selected ones ofsaid plurality of subcarriers, and a unit for copying said onesynchronization signal in the time domain to provide a synchronizationsignal with periodicity.
 20. Apparatus for generating a synchronizationsignal by using a plurality of subcarriers for an OFDM transmissionsystem, comprising: a unit for generating a predefined symbol sequencehaving pre-selected symbols each set to a non-zero value and a pluralityof further symbols each set to a zero value, wherein respective ones ofsaid pre-selected symbols have the value${\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} + j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{+ 1} - j} \right)},{\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} + j} \right)},\quad{{or}\quad\sqrt{\left( \frac{1}{2} \right)}\left( {{- 1} - j} \right)},$ and wherein each of said symbols is mapped respectively on a predefinedsubcarrier of said plurality of subcarriers, a unit for generating onlyone synchronization signal in time domain by performing Inverse FourierTransforming on said plurality of subcarriers mapped with saidpredefined symbols sequence, and a unit for copying said onesynchronization signal in the time domain to provide a synchronizationsignal with periodicity.